1. Field of the Invention
The invention relates generally relates to organic light emitting devices (OLEDs). More specifically, the invention is directed to OLEDs having long lifetimes, including blue devices.
2. Related Art
Opto-electronic devices that make use of organic materials are becoming increasingly desirable for a number of reasons. Many of the materials used to make such devices are relatively inexpensive, so organic opto-electronic devices have the potential for cost advantages over inorganic devices. In addition, the inherent properties of organic materials, such as their flexibility, may make them well suited for particular applications such as fabrication on a flexible substrate. Examples of organic opto-electronic devices include organic light emitting devices (OLEDs), organic phototransistors, organic photovoltaic cells, and organic photodetectors. For OLEDs, the organic materials may have performance advantages over conventional materials. For example, the wavelength at which an organic emissive layer emits light may generally be readily tuned with appropriate dopants.
OLEDs make use of thin organic films that emit light when voltage is applied across the device. OLEDs are becoming an increasingly interesting technology for use in applications such as flat panel displays, illumination, and backlighting. Several OLED materials and configurations are described in U.S. Pat. Nos. 5,844,363, 6,303,238, and 5,707,745, which are incorporated herein by reference in their entirety.
One application for phosphorescent emissive molecules is a full color display. Industry standards for such a display call for pixels adapted to emit particular colors, referred to as “saturated” colors. In particular, these standards call for saturated red, green, and blue pixels. Color may be measured using CIE coordinates, which are well known to the art.
One example of a green emissive molecule is tris(2-phenylpyridine) iridium, denoted Ir(ppy)3, which has the structure of Formula I:

In this, and later figures herein, the dative bond from nitrogen to metal (here, Ir) as a straight line is depicted.
The limited operational stability of organic light emitting devices (OLEDs), however, presents a challenge to their wide-spread acceptance for use in large-area displays and solid-state lighting. While improved packaging techniques and material purity have lead to significant progress in eliminating extrinsic sources of degradation, the remaining intrinsic luminance loss and voltage rise accompanying long term device operation are not yet well understood.
Various hypotheses have been offered to explain the basis for intrinsic degradation in device efficiency, with the most widely accepted advocating chemical degradation of a fraction of the emissive constituent molecules. Presumably, bond cleavage produces radical fragments, which then participate in further radical addition reactions to form even more degradation products. These products act as non-radiative recombination centers, luminescence quenchers, and deep charge traps. For example, several studies have shown that both anions and cations of tris(8-hydroxyquinoline) aluminum (Alq3) are unstable, and evidence has recently been presented that the excited states themselves may form reaction centers in the case of the common host material 4,4′-bis(9-carbazolyl)-2,2′-biphenyl (CBP).